Add ability to 'Attend' an event instance

[Matt Lindsey]

Add 'Attend' button to each event instance that (if you're logged in) adds you to the list of attendees.
Display the list of attendees on the event instance.

Feature: Attend Events
As a site user
In order to indicate to everyone that they will be at an event
I would like to add myself to the Attendee list

Also add to the show_event.feature file to check that, given a few users are signed up to attend an event, their names show up in the Attendees list.

Note that there is already some 'RSVP' functionality for the creator of an event to indicate that they can't actually attend the event. This might be useful to allow people who are 'signed up' for an event to later indicate they can't attend.